When mountain biking, it is important to have a few essential tools with you in case of any mechanical issues or emergencies. Some of the tools you should consider carrying with you include:

• A spare inner tube or patch kit, in case you get a flat tire.  Make sure you get the correct size for your wheel size.  27 or 29
• A tire lever, to help you remove and install your tire.
• A multitool, with a variety of hex keys, screwdrivers, and other tools for making adjustments and repairs.
• A small pump, to inflate your tire after changing the inner tube.
• A chain breaker and extra links, in case your chain breaks or becomes damaged.
• A first-aid kit, to treat any minor injuries you may sustain while riding.
• A map or GPS device, to help you navigate and stay on track.

It is also a good idea to carry some basic survival gear, such as a whistle, a small flashlight, and some high-energy snacks. These items can be helpful in case you get lost or stranded on the trail.

In addition to these tools, you should also make sure to bring plenty of water and food to fuel your ride. It is also a good idea to let someone know where you are going and when you expect to be back, so they can alert authorities if you do not return on time. Mountain biking can be an exciting and rewarding activity, but it is important to be prepared and stay safe while on the trails.